Mount Nyiragongo, in the Virunga mountains of the eastern Democratic Republic of Congo, holds the largest permanent lake of molten rock on Earth, and you can walk up to its rim in a day and sleep beside it. The lake is permanent because of what the lava is made of. It is so poor in silica that it runs almost like water, where ordinary lava crawls at about walking pace, so it never thickens enough to set into a floor. That is also what makes it lethal. Twice in living memory, in 2002 and again in 2021, the lake has drained through cracks in the flank and reached Goma, a city of two million at the foot of the mountain, within hours. The danger is not only the lava. Pockets of odourless carbon dioxide called mazuku settle in hollows around the volcano and have suffocated livestock and children.
Rangers take you up from equatorial rainforest, five or six hours across fields of old lava gone black and hard and then up slopes of loose ash that give under your boots. It asks for stamina rather than skill. At 3,470 metres the ground opens onto a crater whose floor lies a kilometre below the rim, dull grey the whole day. After dark it stops being grey. The lake pulses red down there, hissing and spattering and cracking that whole distance below you. Simple A-frame cabins stand a few metres back from the drop. It is bitterly cold up there, and almost nobody goes to bed early. Dawn hands over the view the night kept from you: cloud lying in a sea below, Congolese forest running green to the horizon, the neighbouring Virunga cones taking the first sun. The climb runs only when the park is open, and the fighting around Goma has closed it. Nothing about the lake pauses for the night; you lie in the dark listening to rock that has never been allowed to harden.